1. Field of the Invention
The present disclosure relates to techniques for measuring parameters of hydrophones, such as techniques for determining acceleration compensation of hydrophones. Although a general area of applicability is listed, other areas not expressly defined may also fall within the intended and implied scope of the present disclosure.
2. Background of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section.
Geologic structures are mapped by creating seismic waves from an artificial source and recording data from the seismic waves with receivers in a borehole. The data comprises arrival time and shape of the waves reflected from acoustic-impedance contrasts. For example, a check-shot survey uses the direct path from source to receiver to measure travel times, whereas a Vertical Seismic Profile (VSP) survey uses the reflected energy recorded at each receiver position, as well as the direct path from source to receiver. For example, in an offshore environment air-guns may be used as the source of acoustic energy. The air-gun source and a hydrophone(s) are deployed over the side of a rig and positioned a few meters below the sea surface. The air gun source typically generates a signal bandwidth from 5-250 Hz.
Hydrophones, as the name suggests, are used to record/measure energy or pressure waves propagating through the sea. As such, hydrophones are used in many applications, including use in proximity to an air-gun to measure the energy of an acoustic source. The acoustic source may be towed in an array behind a boat or positioned in proximity to a marine borehole. However, in each of these environments, a certain amount of noise exists which is caused by various phenomena. Noise affects the quality of the measurements in a negative way. One such example is noise due to the external acceleration forces acting on hydrophones. This acceleration may be due to movement of water past the hydrophone as it is pulled in a streamer behind a vessel, movement of water past the hydrophone suspended beneath a buoy, movement of fluid past the hydrophone in a wellbore, or due to various other movements which create acceleration forces. Noise due to acceleration may be dealt with by using an acceleration canceling hydrophone employing various techniques, including passive and active compensation to accomplish the “acceleration canceling”.
To test the “acceleration canceling” quality and performance of hydrophones, the industry has adopted several testing methods. One testing method involves lowering the hydrophone into a deep pool to provide the necessary hydrostatic pressure to replicate actual use in the sea. The replication is necessary because the physical construction of the hydrophone is affected by hydrostatic pressure that, in-turn, affects the performance of the hydrophone and its acceleration canceling feature. Once located in the deep pool, the hydrophone is shaken to replicate the noise while measurements are taken to determine hydrophone performance. The measurements are obtained and compared with measurements from a reference hydrophone or sensor to achieve a control measurement. However, this type of approach to testing is very costly (infrastructure), dangerous, and difficult to implement.
Another testing method involves testing the hydrophone in the air without any hydrostatic pressure being applied. This typically is referred to as the “Shaker” method and includes mounting a hydrophone to a shaker table/mechanism or speaker and measuring the hydrophones response to movement. Although such technique is well known and is able to utilize standard lab equipment, the approach also has many disadvantages. For example, tests conducted in atmospheric conditions do not test the hydrophone in representative working conditions. Therefore, such testing cannot be representative of the working performance of the hydrophone. For example, such testing cannot provide an accurate indication of hydrophone acceleration canceling properties in a fluid medium or marine environment.